A variety of detection methods have been developed in the art to detect chemical and biological substances in the nanomolar range, and such methods are finding increasing utility in analytical applications ranging from health care to waste treatment. One such general method is based on piezoelectric substances, commonly known as piezoelectric transducers, and, to a lesser extent, optical substances. Typical optical methods for detecting components of biological systems are described in European Patent Publication 0 261 642. Typical piezoelectric methods are described in U.S. Pat. Nos. 4,236,893, 4,314,821, and European Patent Publication 0 215 669.
The ability of piezoelectric transducers to detect small changes in mass, viscosity and density at their surfaces has made them particularly useful as analytical tools where the measurement of very small amounts of material must be made in solution. A typical piezoelectric transducer, based on quartz, comprising an AT-cut quartz crystal sandwiched between two metal excitation electrodes, is described in Karube et al. U.S. Pat. No. 4,789,804. In accordance with this patent, the concentration of an analyte in solution was calculated on the basis of the change in resonance frequency of a bulk shear acoustic wave generated therein caused by the weight of an analyte added to a receptor material immobilized on the surface of the quartz-based device.
Although a useful tool in solution environments, conventional piezoelectric and/or optical substances must undergo considerable complex surface modification to provide specificity for the analyte to be detected. In cases where the analytes to be detected are biological in nature, bioreceptor agents (antigens, antibodies or other ligands) must be immobilized somehow on the surface of the piezoelectric substance. Because attachment of such receptor agents directly to the surface of commonly used piezoelectric substances, e.g., quartz, has been found to be generally unsuccessful, a variety of surface modifications have been developed to facilitate immobilization of receptors. One method of surface modification involves the use of silane and heterobifunctional crosslinking agents, e.g., Eigler et al. U.S. Pat. No. 5,077,210.
Piezoelectric transducers utilized for types of liquid analyses, such as viscosity measurement of an unknown material, involve the generation and propagation of a bulk shear acoustic wave into the unknown material, usually a liquid, see, for example, G. Harrison et al., "Methods of Experimental Physics" Volume 19, Chapter 3, pages 137-171. Due to the low frequency of the bulk shear acoustic wave and large propagation losses in the liquid, the sensitivity of the measurement is limited. Recently, surface elastic waves such as Rayleigh, Bleustein-Gulyaev, and Love waves have been used to sense (measure) parameters of a overlay thin film (liquid) because of the higher sensitivity of the measurement using such waves, see, for example, Hank Wohltjen, Sensors and Actuators, 5, 307-324, 1984; Yu A. Kosevich and E. S. Syrkin, Sov. Phys. Acoust. 36 (1), 8-30, January-February 1990; P. Kielczynski and R. Plowiec, J. Acoust. Soc. AM. 86(2), 818-827, August 1989. Although surface acoustic waves (SAW) possess high sensitivities to the vapor phase sensing applications, they are not adequate for liquid sensing applications, such as biosensing and viscosity measurements, because of large radiation losses to the overlay liquid. See, for example, A. J. Ricco and S. J. Martin, Appl. Phys. Lett, 50(21), 1474 (1987). Therefore, in addition to the SAW waves, it is important to have shear modes available which do not suffer large radiation loss into the liquid so that complete mechanical properties such as elastic modules and viscosity of the overlay material can be obtained. It is a tremendous advantage for a material being able to support surface elastic modes which contains the desired particle velocity components for measurement and sensing applications.
This invention provides a composition that is particularly useful for applications such as biological immunosensing or polymeric viscosity sensing, based upon acoustic or optical changes in the composition.